Traditional input/output (I/O) driver impedance calibration circuits normally experience parasitic external resistance in the pin and package distribution network. This parasitic resistance incorporates a resistance on each side of an external precision target resistor. However, this parasitic resistance is usually limited to a small value.
However, for a 1:1 calibration architecture with a low impedance and a tight tolerance, the parasitic external resistance variation may consume a large amount of the tolerance budget. For example, if the impedance is 40 ohms, and there is a tolerance of 10% (±4 ohms), a small resistance of 1.5 ohms on either side of the external precision target resistor (total of 3 ohms) takes up 3.8% of the 10% tolerance.